Generally, in fabrication of an IC device, a photolithography (lithography) process may be utilized to print/pattern various layers of a circuit design onto a surface of a silicon (Si) substrate for creating various devices (e.g., transistors) and circuits to form the IC device. In lithography, a photomask is used to mask or expose areas on the substrate that are to be blocked from or patterned by, respectively, a light source (e.g., EUV). With miniaturization of the IC devices, EUV lithography (e.g., with 13.5 nm wavelength photons) is utilized to achieve a better resolution when compared to other lithography options (e.g., 193 nm immersion lithography, multiple patterning, etc.). Optical elements in an EUV scanner must be reflective, which require that the EUV photomask be illuminated at an angle to its normal (non-orthogonal). However, non-orthogonal illumination of the EUV photomask can cause, for example, a shadowing of lines that are perpendicular to the incident beam, the appearance of telecentricity errors that can result in a pattern shift through focus, and image contrast loss due to apodization by a reflective mask coating.
FIG. 1A is a cross sectional diagram of an EUV photomask. Diagram 100 illustrates a portion of an example EUV photomask including a multilayer reflector (MLR) stack 101 of alternate layers of molybdenum and silicon material, a ruthenium (Ru) capping layer 103, layer 105 including EUV absorber material, and antireflective coating (ARC) layer 107. An EUV beam 109 is at angle 111 of six degrees (6°) that causes reflective beams 113 to reflect from effective reflection plane 115. However, the off-axis illumination of the EUV beam 109 can cause a shadowing effect due to the thickness of the layers 105 and 107. For example, there can be a shadowing at the capping layer 103 as the EUV beam 109 passes by the ARC layer 107 at contact point 117. A lower profile of the combined thickness of the layers 105 and 107 could provide for less interference with illumination of the EUV beam 109 onto the upper surface of the Ru cap 103 and the reflective reflection plane 115. Even in a scenario where there is no ARC layer, the contact point 117 may be at the upper surface of the absorber layer 105. However, as in diagram 150 of FIG. 1B, in order to maintain the EUV reflectance 151 at or below 2%, the thickness 153 of a typical tantalum (Ta) based absorber layer is about 50 nm to 60 nm. Tantalum nitride (TaN) or tantalum boron nitride (TaBN) are the industry standard for EUV photomask absorber layers for their etch compatibility even though Ta-based materials have only a moderate EUV absorption coefficient. Although other materials such as nickel, cobalt, sliver, etc. with high EUV absorption coefficients may be used to achieve a thinner absorbing layer 105, such materials cannot be etched away during the fabrication process of an IC device without affecting/damaging adjacent layers, e.g., the MLR stack 101.
A need therefore exists for a methodology enabling creation of an EUV photomask with an etchable and highly absorbing thinner absorber layer and the resulting photomask.